VANCOUVER — Geophysical surveys are frequently used by the mining and exploration industry to detect potential mineral deposits hidden beneath the subsurface. However, the results of geophysical surveys can be confusing or misleading to retail investors and nonspecialists, with data often painted as an ambiguous red bull’s-eye on company maps.
And so The Northern Miner reached out to Greg Hodges, a senior geophysicist at Sander Geophysics — an international airborne geophysical survey firm for more than 60 years — to discuss what investors need to know about geophysics, and how to separate the robust geophysical anomalies from the not-so-robust ones.
Here are the top-seven questions Hodges says nonspecialists could ask companies to gain a better understanding of their geophysical survey results.
1. Does the company have a geological model?
Geophysical surveys measure the magnetic response, resistivity and conductivity, density, chargeability, radioelements and seismic velocities of rocks using an array of techniques. These fluctuations in “rock properties” are caused by changes in geology, such as varying rock compositions, faulting, alteration or mineralization.
In an ideal geological setting, a deposit can have a predictable geophysical signature. However, Hodeges reminds us that geology is rarely ideal, and a deposit’s geophysical signature can change depending on the rocks that surround it.
“You need to know the geology of an area before you can predict what kind of geophysical response a target would have — you could be after a geophysical high [normally red on the map] or low [blue], depending on the target and the surrounding geology,” he says.
He says that diamond-bearing kimberlites in the Lac de Gras region in the Northwest Territories are weakly conductive in geophysical electromagnetic (EM) surveys, whereas at DeBeer’s Victor diamond deposit in northern Ontario, there’s no contrast in the EM data because the kimberlites are hosted in equally conductive rocks.
“If the host rock measures 300 ohm in a survey and the kimberlite measures 320 ohm, then that’s very little contrast. The kimberlite won’t stand out in the data,” he says.
Investors can assess the credibility of geophysical data by asking whether the project’s geology could interfere with the size and magnitude of the geophysical anomaly.
2. Is there more than one type of geophysical survey that suggests the anomaly is a potential deposit, rather than variability in the geology?
The most common geophysical surveys used in the industry are induced polarization (IP), EM, magnetic, gravity and radiometric surveys.
In an ideal geological environment, IP is the best method for low sulphide systems, such as porphyry copper-gold deposits.
EM surveys measure the electrical conductivity and resistivity of a rock, and can be used to explore for deposits with a high sulphide content, like magmatic-sulphide or volcanogenic massive sulphide (VMS) deposits.
Magnetic surveys measure the amount of magnetic minerals in the subsurface, and can be used to highlight iron oxide copper-gold deposits or iron formations that may carry gold. A gravity survey measures the density of the subsurface and is useful for massive sulphide deposits and intrusions, and determining structure.
Radiometric surveys measure the uranium-thorium-potassium values of rocks at surface and can be useful for mapping, highlighting geological contacts or alteration — as long as the rocks at surface are the same as the bedrock, Hodges points out.
For Hodges, just one type of geophysical anomaly isn’t enough to give credence to a company’s exploration target, because geology is usually too variable.
“If they’re focused on just one anomaly they have to be very confident about what they’re looking at,” he says. “Every survey you have adds some information, or reduces the chances of having two things that look alike.”
As an example, explorers on the hunt for porphyry deposits may look for correlatable “circles” in magnetic and electromagnetic surveys. Within a porphyry system, the intrusive can be a magnetic high or low and is normally resistive in geophysical surveys, whereas the halo of alteration around it could be relatively conductive.
“If you have a metal-rich supergene blanket on top of the porphyry then now you have a wonderful conductive electromagnetic anomaly on top of it,” he says. “Again, what you’re looking for goes back to the geological model — the geophysics can change depending on the target and the geology of the area.”
3. Is there enough geophysical data to show the anomaly is there, and is it unique?
Most junior companies can’t afford a large, regional geophysical survey, but surveys completed over a very small area may not provide explorers with enough data, Hodges says.
“A company could have a little wee map with a big red blob on it, but if they had more coverage you’d see red blobs everywhere — there’s nothing unique about it. You want to see what geophysicists call ‘background’ around the target to confirm that the target is different from the rest of the geology.”
The resolution of the survey is equally important. Resolution is defined by the spacing between measurements on a line, and can range from 10 to 250 metres, or more. The distance between each measurement point, and the overall size of the survey area, dictate how deep and clearly the geophysical survey can “look” into the subsurface.
“If you’re after a small, narrow deposit you have to sample small enough to see it,” he says. “If you’re sampling along geophysical stations that are 25 metres apart, each station is measuring a block of rock 50 metres horizontally and 50 metres vertically — that’s your resolution.
“If you want to see deeper, you need bigger arrays and sample bigger volumes of rock, so maybe you’ll measure more widely spaced stations on lines that are 500 metres apart. In this case, a single measurement will take in a lot more geology than a 25-metre spaced survey, which means that any target smaller than your resolution would be very difficult to see or impossible to define.”
Sparse data over an anomaly — such as surveys where only one line crosses the target — may not be enough to warrant follow-up drilling, he adds.
“My rule of thumb is at least two lines of data over the target. If they just did one line, that’s nice, but can they guarantee that if they go over it again we’d see the same things? Or did something funny happen in the data that’s not important?”
Hodges notes that companies “want to see everything in a survey, but that’s expensive. They need to consider the economic aspects of the target they’re going after — that is, the size and depth. At that point we can design a survey that can see a target that fits their criteria with reasonable constraint, and save them unnecessary costs.”
4. How does the magnitude of the anomaly compare to other anomalies or deposits?
Retail investors and non-specialists may get distracted by the vivid colours presented on a company’s geophysical map, but Hodges reminds us that appearances can be misleading.
“Companies can shade an anomaly from standout red to background green simply by adjusting the colour scale,” he says. “There’s so much you can do with colour, you can hide things or make things show.”
He adds that every geophysical image needs to be accompanied with a colour bar that displays the magnitude of the geophysical readings, so other geoscientists or savvy investors can gauge the intensity of the anomaly by comparing it with other anomalies on the property, or nearby deposits that are geologically similar.
In other circumstances, interesting anomalies could get lost in the data stretch, he adds.
For example, on a magnetics map, a large batholith may be highlighted as a bright pink colour — indicating high magnetic intensity — but the sought-after, more subtle magnetic anomalies could fall into background values and be painted blue.
“The best target might not be the biggest red bump on the map,” Hodges says.
5. Is the geophysical anomaly shaped like a deposit?
Good geophysical data “looks” like geology, with folds, faults and pinch-outs.
“Robust geophysical anomalies should have a shape that fits the geological model of the deposit,” Hodges says. “You can have kilometre-long conductive anomalies in a meta-sedimentary basin, for example, but you know those aren’t VMS deposits — it doesn’t have the right shape. VMS deposits don’t go on for kilometres, they’re more like 200 metres in length. The anomaly may be more consistent with large belts of graphitic sediments, which are also conductive.”
6. Has the geophysical data been processed by qualified geophysicists?
A company can better interpret its geophysical results, avoid false anomalies and highlight the more subtle ones by processing the raw geophysical data into a 2- or 3-D model that highlights the varying rock properties, Hodges says.
This processing technique, called inversions, can be constrained or unconstrained. Constrained inversions use other geophysical data and/or geological criteria — such as drill holes, surface mapping or conceptual models — to produce a reasonable geophysical model. An unconstrained model is produced without any intervention from other data sources.
Inversions are proving to be a powerful tool in geophysics, and companies can purchase advanced computer software to perform them in-house.
But Hodges warns that inversions can be “difficult to perform, and you need experts to have it done right.
“If you make the wrong assumption about the geology you may force the geophysical data towards something you expect, but doesn’t fit the data or the real world,” he says.
“The more geophysical and geological data you have, the better you can constrain the model.”
The more a company has to process its geophysical data to “pull out” the anomalies without much data to support the model, “the more nervous I get,” Hodges says.
7. Did a reputable geophysical company conduct the survey and is it a well-known geophysical method?
Hodges uses the term “voodoo geophysics” to describe the work of some geophysical consulting firms that make “overzealous” claims on the capabilities of a dubious new technology.
“There was a company some years back that claimed they could see a deposit the size of a Volkswagen at a few kilometres depth, and every geophysicist worth his salt scoffed because they know that can’t happen — resolution falls off at depth,” he says.
Another firm claimed to see diamonds from satellite data using proprietary software, despite diamonds being virtually undetectable in geophysics.
“Companies should only use geophysical techniques based on sound science that are backed by reputable and experienced geophysical firms,” Hodges adds. “If it’s a one-off company doing something no one else would do, you have to wonder why.”